1. Field of the Invention
This invention relates to pacers for the therapeutic stimulation of the heart.
2. Description of the Prior Art
Pacers for stimulating the heart are well known in the art. Traditionally these devices have been manufactured utilizing discrete analog circuitry of limited complexity. More recently, however, pacers have been designed and manufactured with integrated digital circuitry of great complexity. This additional capability has been used to add desirable features such as telemetry and programmability to these modern pacers.
The programmability feature permits the attending physician to noninvasively alter a pacer parameter such as pacer rate. Typically, the value of the selected parameter is stored locally within the pacer in a volatile semiconductor memory. One problem associated with pacers operating under the control of locally stored data relates to failure modes resulting from the loss of this stored information. These software driven pacers are susceptible to unintentional memory change or phantom programming caused by electromagnetic interference. This interference may result in an alteration in memory contents and result in pacing at a rate substantially different from that previously programmed into the pacemaker by the attending physician. As a consequence, these software related errors may result in pacer-induced bradycardia or pacer-induced tachycardia which forces the heart to operate outside physiologically safe limits.
One prior art solution directed to the problem of pacer induced tachycardia is taught by U.S. Pat. No. 3,391,697 to W. Greatbatch. One embodiment taught by this patent involves the use of circuitry interposed between the oscillator rate determining portion of the pacemaker and the output portion of the pacemaker. In operation the rate limit system prevents stimulating pulses from being delivered to the heart above a preset upper frequency limit. This form of rate runaway protection has been widely adopted and prevents single component failures in the pacer from producing a life-endangering rate runaway condition.
Although this technique has been widely adopted in modern digital pacemakers, it does not address the low rate failure mode nor the other problems faced by pacers operating under the control of stored data.
A further prior art patent which relates generally to rate limit techniques for pacers is the U.S. Pat. No. 3,903,897 to Woolons, et al., which describes generally an A-V sequential pacer with upper and lower rate limit circuits. In this pacer, synchronizing pulses derived from the atrial or ventricular cardiac depolarization are ignored beyond the preset upper and lower rate limits. If these rate limits are exceeded, asynchronous pacing pulses are produced in a reversion mode. This rate limit technique does not address nor is it applicable to the problems posed by pacers operating under the control of stored data but is concerned only with the effect on the pacer of several depolarizations exceeding the upper rate limit or falling below the lower rate limit.
Pacers operating under the control of stored data are known in the art from commonly assigned, copending U.S. patent application Ser. No. 957,959 filed Nov. 6, 1978 now U.S. Pat. No. 4,253,465. Programmable pacers of this type have a number of critical operating parameters stored in a volatile semiconductor memory. The loss of this information through phantom programming or through other means may result in output pulses being delivered at a rate outside of physiologically safe limits.